This invention relates to the preparation of fine xcex2-silicon carbide powder useful as heat resistant material, pyrogenic material, structural material, and abrasive material.
Several methods are known in the art for the preparation of silicon carbide powder. Typical methods include (1) reductive carbonization of silica and carbon, (2) direction reaction of metallic silicon with carbon, (3) pyrolysis of organosilicon compounds, and (4) gas phase reaction.
Of these methods, the reductive carbonization method (1) and the direct reaction method (2) produce silicon carbide powder composed mainly of coarse particles. When this silicon carbide powder is used as sintering material or in abrasive application, it must be ground and classified to a finer size, which steps require much labors. Even after such grinding, the resulting silicon carbide powder is not yet regarded adequate to the relevant application. The gas phase reaction method (4) starts with such raw materials as SiH4, SiCl4 and CH4 which are expensive. Since the current technology is low in yield, the product powder is costly. For the most part, the silicon carbide powder obtained by this method is used only in the thin film and single crystal-forming application and not suited as industrial powder materials. As a consequence, the method (3) involving pyrolysis of organosilicon compounds is considered most potential for the preparation of silicon carbide powder, with a number of studies having been made thereon. For example, JP-B 1-42886 discloses a method for preparing silicon carbide powder by uniformly liquefying a liquid silicon compound, a liquid organic compound having a functional group and capable of forming carbon upon heating, and a polymerization or crosslinking catalyst, effecting polymerization or crosslinking reaction to form a precursor containing Si, O and C, and heating the precursor in a non-oxidizing atmosphere. In JP-A 61-6110, silicon carbide powder is prepared by adding a siliceous solid and a carbonaceous solid to a liquid silicon compound and an organic compound having a functional group to form a precursor solid, followed by firing and carbonization of the precursor solid. The method of JP-A 6-183718 involves furnishing a liquid silicon compound and a liquid organic compound having a functional group and capable of forming carbon upon heating as starting materials, adding a polymerization or crosslinking catalyst thereto, effecting polymerization or crosslinking reaction to form a precursor, and firing in two stages the precursor in a non-oxidizing atmosphere, thereby producing xcex2-type silicon carbide powder.
The above-referred methods relying on pyrolysis of organosilicon compounds, however, are still not satisfactory from the industrial standpoint. More particularly, the method of JP-B 1-42886 uses a silane or siloxane having a functional group (e.g., hydrolyzable group such as alkoxy) as the starting material. An organic compound having a functional group, typically a phenolic resin giving a large amount of residual carbon is used as the carbon source. The silane or siloxane and the organic compound are subjected to heat polymerization or catalytic polymerization, followed by heating in a non-oxidizing atmosphere. Silicon carbide is produced in two steps. Basically, this method produces silicon carbide by way of silicon dioxide. Even if silicon carbide is ideally produced most simply by reacting silicon dioxide with carbon as shown by the following scheme, the yield based on the reactants (SiO2 and C) is only 42% at the maximum.
xe2x80x83SiO2 (s)+C (s)xe2x86x92SiO (g)+CO (g)
SiO (g)+2C (s)xe2x86x92SiC (s)+CO (g)
Actually, while reaction proceeds as shown by the scheme, a substantial loss occurs with SiO (g) having a high vapor pressure, resulting in a further reduced yield. If the yield is calculated based on the starting material, silane or siloxane, the yield is apparently far below this level. Additionally, this method is cumbersome in that it requires two stages, polymerization step and firing step for carbonization into silicon carbide.
The method of JP-A 61-6110 proceeds by way of a liquid compound resulting from acidolysis or alkali-removal reaction of an aqueous alkali silicate solution or an esterified product of the liquid compound with an organic compound having a hydroxyl group such as a phenolic resin. This is mixed with silicon dioxide and organic coke powder to form a solid which is fired. In this sense, the method is basically identical with the above-discussed method of JP-B 1-42886 and suffers from low yields and cumbersome operation.
The method of JP-A 6-183718 is identical with the foregoing methods in that a mixture of an organosilicon compound such as tetraethoxysilane and an organic resin such as a phenolic resin as a reducing agent is used as the starting material, and a condensation product thereof is formed as the precursor. This method is distinguishable in that silicon carbide having a higher purity can be produced by using a high purity silane for semiconductor use as the starting material and adding a halide such as hydrogen chloride during the firing step. It suffers from low yields and cumbersome operation as well.
An object of the invention is to provide a method for preparing xcex2-silicon carbide powder through simple steps, at a low cost and in high yields.
Surprisingly, we have found that by impregnating graphite with at least one organosilicon compound selected from crosslinkable silanes and siloxanes, forming a crosslinked product of the organosilicon compound within the graphite, and heating at a temperature of 1,300xc2x0 C. or higher in an inert gas stream for reaction, dense silicon carbide is produced in a high yield based on the starting material (i.e., organosilicon compound).
More specifically, according to the inventive method, the liquid or gaseous organosilicon compound having functional groups is converted into a high density crosslinked structure having silicon-to-carbon bonds through hydrosilylation or similar reaction, thereby overcoming the problem inherent to the use of such organosilicon compounds as the starting material that the organosilicon compound will volatilize during firing in a non-oxidizing gas stream. Establishing an ideal production route using carbon itself as a reducing agent and as a carbon source for silicon carbide, we have arrived at the present invention.